Fluid catalytic cracking (FCC) is a petroleum refining process, which has been applied commercially on a large scale. The FCC feedstocks usually contain sulfur in the form of organic sulfur compounds, e.g. mercaptans, sulfides and thiophenes. The FCC products correspondingly contain sulfur impurities, which were induced into the gasoline fraction in some ratio and then into the gasoline blending pool. With the increasing attention to the environment recently, the limitation of the sulfur content in gasoline fraction is becoming more critical. The limitation is not only advantage to the environmental protection but also important for decreasing the poison degree of sulfur in the catalytic transformer of automobile.
The traditional desulfurization is hydrogenation process used to remove a part of sulfur compounds to decrease the sulfur content of gasoline. The FCC feedstocks can be hydrogenated before cracking alternatively; and the cracked products can be hydrogenated after FCC process. The former is expensive in terms of the capital cost of the equipment and the operation because of the high hydrogen consumption. The later tends to saturate the olefins in the products leading to the lost of octane value.
From the economic point of view, it would be desirable to remove sulfur in the FCC process itself without additional treatment. In order to fulfill this target, some research works have centered on the removal of sulfur from the regenerator stack gases, but actually the sulfur levels of cracking products are not greatly affected through this method (Krishna et. al., Additives Improve FCC Process, Hydrocarbon Processing, 1991, 11, 59-66); other works are removal of sulfur from gasoline directly in FCC process by adding additives with desulfurization properties.
The investigations on the additives with desulfurization properties indicate that some metal elements such as V, Ni, Cu, Cr, Sn, B, Al and Zn etc. have such properties. For example, a desulfurization catalyst GFS and a desulfurization additive GSR for decreasing the gasoline sulfur have been developed by Grace Davison. The desulfurization additive disclosed in U.S. Pat. No. 5,376,608 and U.S. Pat. No. 5,525,210 is a kind of Al2O3 materials loaded with metals, in which the metal component could be selected from Ni, Cu, Zn, Ag, Cd, In, Sn, Hg, Tl, Pb, Bi, B and Al. The Al2O3 materials loaded with 5˜10 wt % V disclosed in U.S. Pat. No. 6,482,315 show better desulfurization activities from gasoline when used in combination with FCC catalysts contained a zeolite Y as active component.
Chinese patent CN1261618A disclosed a method used for the removal of gasoline sulfur in FCC process. The composition of desulfurization catalysts in this method comprises a porous molecular sieves component, normally a faujasite such as zeolite USY, a metal in an oxidation state above zero value deposited within the pore structure of the sieves, preferably the vanadium, as well as a rare earth component employed to increase the cracking activities.
In catalytic cracking process the FCC catalysts are always used at high temperatures and need a recycle process such as steam stripping and oxidation-regeneration steps. So, the stability especially the hydrothermal stability of the FCC catalysts is very important. The cracking catalysts with desulfurization property mentioned above are formed by supporting metal compounds on matrix. The metal ions are present as exchanged cationic species, which leads to the decrease of hydrothermal stability of the catalysts. At high temperatures the supported metal component could transfer to the molecular sieves and lose desulfurization property. Especially, the transferred metal component vanadium will destroy the structure of the active molecular sieves and result in the deactivation of the FCC catalysts.